Exam 2 · Adaptations / Sex / Social Study Guide

BIOL 4230 · Evolution · Exam 2 · Adaptations / Sex / Social — Final exam Mon May 4, 2026 · 5–7 PM · Dr. Travis Robbins
Lectures: L07 · L08 · L09 · L11 · L12 · L13

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L07 · Empirical Studies of Natural Selection (Ch 8)

How biologists actually MEASURE natural selection in the wild — landmark long-term studies that move 'selection' from theory to observation. Grants' finches, peppered moths, and similar systems are the empirical canon.

§A — How selection is measured in nature

To demonstrate natural selection in the field, you need to (1) measure heritable variation in a trait, (2) measure differential survival or reproduction associated with that variation, and (3) ideally see allele-frequency change across generations.

Key points

• Field measurement of selection requires longitudinal data — tracking marked individuals across years.
• Selection differential S can be measured directly by comparing the mean trait of breeders to the population mean.
• Heritability h² can be estimated from parent-offspring resemblance in the wild.
• Predicted response R = h² · S can be checked against the observed change in the next generation.

Key terms

• Selection in the wild — Demonstrated when individuals with certain heritable trait values reproduce more than others in a natural setting.

§B — Grants' Galápagos finches

Peter and Rosemary Grant's decades-long study of medium ground finches (Geospiza fortis) on Daphne Major is the gold standard of natural selection in action.

Key points

• After the 1977 drought, large hard seeds dominated the food supply. Birds with deeper, stronger beaks survived better.
• The trait (beak depth) was heritable; the next generation had measurably deeper beaks — natural selection caught in real time.
• Subsequent wet years reversed the selective pressure (smaller seeds favored), demonstrating that selection direction is environment-contingent.
• Hybridization with a related species also introduced variation, showing how gene flow and selection can interact.

Key terms

• Geospiza fortis — The medium ground finch on Daphne Major, focus of the Grants' long-term study.
• Selection on beak depth — After the 1977 drought, deeper-beaked birds survived better and the next generation had deeper beaks on average.

§C — Peppered moths and industrial melanism

Peppered moths (Biston betularia) shifted from light to dark forms during the Industrial Revolution and back again with pollution control. The textbook case of rapid microevolution under human-caused selection.

Key points

• Pre-industrial: light morph dominant; rested camouflaged on lichen-covered tree trunks.
• Industrial-era pollution killed lichen and blackened tree trunks; dark (melanic) form had a survival advantage and rose to >95% in polluted regions.
• Selection mechanism: differential bird predation on visible morphs.
• After clean-air legislation reduced soot, lichen returned, and the light morph rebounded — selection direction reversed.
• The genetic basis is now known: a transposon insertion in the cortex gene.

Key terms

• Industrial melanism — Increase of dark (melanic) morphs in animal populations due to human-caused environmental darkening; classic in Biston betularia.
• Biston betularia — The peppered moth species in which industrial melanism was documented.

§D — Other documented examples — flu, antibiotics, domestication

Beyond classic natural systems, human activities provide many real-time tests of selection: pathogens evolving resistance, domesticated species evolving under artificial selection.

Key points

• Influenza evolves antigenic variation rapidly under selection from host immune systems and (where used) antiviral drugs.
• Antibiotic resistance in bacteria is a textbook case of strong, directional natural selection by humans.
• Domestication is artificial selection — humans choose breeders. Greyhounds bred for speed show measurable evolution of sprint biomechanics over decades.
• These cases share a structure: heritable variation + strong directional selection → rapid measurable change.

Key terms

• Antibiotic resistance — Evolution of bacteria able to survive antibiotic exposure; driven by selection on rare resistant variants in large populations.
• Artificial selection — Selection in which humans (rather than the natural environment) determine which individuals reproduce.

L08 · Complex Adaptations (Ch 10)

How complex traits — eyes, wings, body plans — evolve through stepwise mutations, regulatory changes, and gene-network conservation. This lecture is the EVO-DEVO chapter, integrating molecular biology with classical evolutionary thinking.

§A — Adaptation as both trait and process

The word 'adaptation' has two meanings: a trait that has been shaped by past selection, AND the process by which selection produces such traits. Complex adaptations build incrementally — they do not appear in their finished form.

Key points

• Adaptation (noun) = a heritable trait whose current form is the product of past natural selection in service of some function.
• Adaptation (verb) = the process by which selection improves fit between organism and environment.
• Complex adaptations arise through MANY small mutational steps, each conferring some fitness advantage along the way.
• Each intermediate stage must itself be functional and selectively advantageous — selection cannot 'plan ahead.'

Key terms

• Adaptation — Either a trait shaped by past selection (noun) OR the process producing such traits (verb).
• Gradual evolution — Complex traits accumulate from many small mutational changes, each itself favorable.

§B — Evolution of the vertebrate eye — gradual stepwise model

The vertebrate eye is the textbook example of complex adaptation evolving through functional intermediates. Each step (light-sensitive patch → cup → pinhole → lensed eye) is itself useful.

Key points

• Stage 1: light-sensitive patch — distinguishes light from dark; useful for circadian rhythms and predator detection.
• Stage 2: cupped patch — adds directional sensitivity; can detect WHERE light comes from.
• Stage 3: pinhole eye (e.g., Nautilus) — narrows the aperture, producing a crude image without a lens.
• Stage 4: lens added — focuses light; sharper image. Lenses likely evolved from co-opted crystallin proteins.
• Eyes have evolved INDEPENDENTLY at least 40 times across animals — convergent solutions to a common selective problem.

Key terms

• Stepwise eye evolution — The gradual elaboration from light-sensitive patch to cup to pinhole to lensed eye, each step functional.
• Convergent evolution of eyes — Eyes have arisen independently in many animal lineages; the camera eye of vertebrates and cephalopods is the classic case.

§C — Regulatory networks and gene duplication

A lot of evolutionary novelty comes from RE-WIRING existing genes rather than inventing new ones. Mutations in regulatory regions, gene duplication followed by sub- or neofunctionalization, and changes in expression timing/location all reshape phenotype without changing the underlying protein-coding repertoire.

Key points

• Mutations in cis-regulatory elements (enhancers, promoters) can reshape WHEN and WHERE a gene is expressed without changing the protein.
• Gene duplication produces redundant copies — one can retain the original function while the other evolves a new role (NEOFUNCTIONALIZATION) or a subset of the old role (SUBFUNCTIONALIZATION).
• Many adaptations are 'tinkering' on conserved gene networks rather than de novo invention.
• PROTEIN PROMISCUITY — proteins often have weak side-activities that can be co-opted and refined into new primary functions.

Key terms

• Cis-regulatory mutation — Mutation in a regulatory sequence (enhancer, promoter) that alters when/where a gene is expressed without changing the protein.
• Gene duplication — Production of an extra copy of a gene; relaxed selection on the duplicate enables new function evolution.
• Neofunctionalization — One copy of a duplicated gene evolves a novel function while the other retains the original.
• Subfunctionalization — Both copies of a duplicated gene specialize on different subsets of the ancestral function.
• Protein promiscuity — Proteins often have weak side-activities; selection can refine these into new primary functions.

§D — Heterochrony — changes in developmental timing

Many morphological differences across species reflect changes in WHEN developmental processes occur, not WHAT they do. Speeding up, slowing down, or shifting the onset of a process produces dramatically different adult forms.

Key points

• HETEROCHRONY = evolutionary change in the relative TIMING or RATE of developmental events.
• Examples: paedomorphosis (juvenile features retained in adults — axolotl salamander), peramorphosis (extension of growth past ancestral state — large antlers).
• Small changes in timing (a few hours of extra cell proliferation) can produce dramatic morphological differences.
• Heterochronic changes often have a simple genetic basis — a few mutations in regulatory genes.

Key terms

• Heterochrony — Evolutionary change in the timing or rate of developmental events between species.
• Paedomorphosis — Retention of juvenile features in the adult form (e.g., axolotl reproduces while still in larval form).
• Peramorphosis — Extension of development past the ancestral end-point, producing more derived adult forms.

§E — Hox genes and conserved developmental networks

Hox genes encode transcription factors that pattern the anterior-posterior axis of bilaterian animals. They are deeply conserved across phyla — the same Hox toolkit patterns flies, mice, and humans.

Key points

• Hox genes are clustered in a chromosomal arrangement that mirrors their expression pattern along the body axis (COLINEARITY).
• Hox proteins are transcription factors that turn on segment-specific developmental programs.
• The same Hox genes pattern the body axes of vastly different animals — flies, mice, humans — testifying to deep evolutionary conservation.
• Differences in WHERE Hox genes are expressed (regulatory evolution) underlie many morphological differences across species.
• Hox-cluster duplications correlate with major body-plan transitions (the vertebrate genome has 4 Hox clusters; invertebrates typically have 1).

Key terms

• Hox gene — Member of a family of transcription factors that pattern the anterior-posterior body axis in bilaterian animals.
• Colinearity — The arrangement of Hox genes along the chromosome matches their expression order along the body axis.
• Conservation of developmental networks — Core gene-regulatory networks (Hox, Pax, Wnt) are shared across distantly related animals — evidence of deep common ancestry.

§F — Imperfect adaptation — limits and constraints

Selection produces traits that are 'good enough,' not optimal. Historical legacy, developmental constraints, and trade-offs leave fingerprints of imperfection — and those imperfections are often the strongest evidence of evolution.

Key points

• Adaptations are constrained by history — selection can only modify what's already there.
• Vestigial structures (whale pelvis, human appendix, ostrich wings) reflect ancestral function with no current use.
• Trade-offs — improving one trait often comes at the cost of another (running speed vs. heat dissipation).
• The vertebrate retina has the photoreceptors BACKWARDS (light passes through neurons before reaching them) — a historical accident, not a design choice.

Key terms

• Vestigial structure — A structure that retains ancestral form but has lost most or all of its original function.
• Trade-off — Improving one trait comes at the cost of reduced performance in another.
• Constraint — Historical or developmental factors that limit what selection can produce.

L09 · Coevolution (Ch 15)

Coevolution is reciprocal evolutionary change in two interacting species — each lineage's adaptations are evolutionary responses to the other. It produces arms races, mutualisms, and the geographic mosaic of selection.

§A — Defining reciprocal coevolution

Coevolution is not just any pair of species coexisting — it requires that adaptations in one lineage drive adaptive responses in the other, in a feedback loop.

Key points

• Coevolution = reciprocal evolutionary change between two (or more) interacting lineages, each responding to selective pressure from the other.
• Coexistence alone is not coevolution; the interaction must shape heritable trait change in both directions.
• The interaction can be antagonistic (predator-prey, host-parasite) or mutualistic (pollinators-flowers).

Key terms

• Coevolution — Reciprocal evolutionary change in two or more interacting species driven by selective pressures each imposes on the others.
• Reciprocal selection — Selection imposed by one species on another, which then imposes selection back on the first.

§B — Antagonistic arms races

When predator-prey or host-parasite interactions persist, each side evolves countermeasures, which selects for new adaptations in the other side, in an open-ended escalation.

Key points

• Classic case: rough-skinned newts (Taricha) produce tetrodotoxin (TTX); garter snakes (Thamnophis) evolve TTX-resistant sodium channels.
• Geographic variation is striking: where snakes are highly resistant, newts are highly toxic; where snakes are sensitive, newts are weakly toxic.
• Arms races can be asymmetric — one side may 'win' temporarily, with the other lagging behind in evolutionary response time.

Key terms

• Coevolutionary arms race — Reciprocal escalation of offense/defense traits in interacting species, driven by ongoing antagonistic selection.
• Tetrodotoxin (TTX) — Potent neurotoxin produced by newts that blocks voltage-gated sodium channels; basis of the newt-snake arms race.

§C — Mutualistic coevolution

Mutualisms are coevolutionary too: each partner's traits are adapted to the other, and selection on one drives change in the other in a cooperative direction.

Key points

• Pollinator-plant mutualisms (e.g., long-tongued moths and long-spurred orchids — Darwin's prediction) show co-adapted morphology.
• Endosymbiosis (mitochondria from ancestral alphaproteobacteria, chloroplasts from cyanobacteria) is the deepest mutualism — now obligate.
• Mutualisms can break down or shift to parasitism if cost/benefit balance changes.

Key terms

• Mutualism — An interaction in which both species gain net fitness benefit.
• Endosymbiosis — One organism living inside another; classic origin of eukaryotic mitochondria and plastids.

§D — Mimicry — Batesian vs. Müllerian

Mimicry is a coevolutionary product where one species' phenotype evolves to resemble another. The two flavors differ in who pays the cost.

Key points

• Batesian mimicry — a HARMLESS mimic resembles a HARMFUL model; mimic gains protection without bearing the cost of being toxic. Mimic must stay rare or predators learn the trick.
• Müllerian mimicry — multiple HARMFUL species converge on a shared warning signal; predators learn 'this color = bad' faster, benefiting all.
• Both are products of frequency-dependent selection by shared predators.

Key terms

• Batesian mimicry — A palatable species evolves to resemble an unpalatable model species, gaining protection from predators.
• Müllerian mimicry — Multiple unpalatable species converge on a similar warning signal, sharing the cost of educating predators.
• Aposematism — Conspicuous warning coloration in unpalatable or dangerous species.

§E — Geographic Mosaic Theory of Coevolution

Thompson's framework: coevolutionary outcomes vary across geography because local ecological conditions tilt the cost/benefit balance differently in different places.

Key points

• Coevolution happens in 'hotspots' (intense reciprocal selection) and 'coldspots' (weak or no reciprocal selection).
• Gene flow between hotspots and coldspots, plus local trait evolution, generates a mosaic of coevolutionary states across the species' range.
• Predictions: trait values mismatched in some places (showing the mosaic), no single 'optimal' coevolutionary outcome.

Key terms

• Geographic Mosaic Theory of Coevolution — Thompson's hypothesis that coevolution proceeds at different rates and in different directions across a species' geographic range, producing spatially variable trait combinations.

L11 · Sex and Sexual Selection (Ch 11)

Why does sex exist at all, given its costs? Once it exists, why do males and females differ so dramatically? This lecture covers the evolution of sexual reproduction, the origin of anisogamy, and the consequences — sexual selection, sexual conflict, sperm competition.

§A — The cost of sex and why sex evolved anyway

Sexual reproduction has obvious costs (the 'twofold cost of males,' searching for mates, recombination breaking up good gene combinations) — yet sex is widespread. The benefits must outweigh these costs.

Key points

• TWOFOLD COST OF SEX (Maynard Smith): in sexual species, males don't directly produce offspring, so a sexual mother passes on only HALF as many genes per offspring as an asexual one would.
• BENEFIT 1 — Genetic diversity through recombination: sexual reproduction shuffles alleles, creating new combinations selection can act on.
• BENEFIT 2 — MULLER'S RATCHET: in asexual lineages, deleterious mutations accumulate irreversibly; sex purges them via recombination.
• BENEFIT 3 — RED QUEEN HYPOTHESIS: parasites coevolve rapidly with hosts; sexual recombination lets hosts 'run faster' to stay ahead of parasites.
• BENEFIT 4 — Sib competition reduction: sexual offspring are genetically diverse, reducing competition among siblings for the same niche.

Key terms

• Twofold cost of sex — Sexual females produce half the gene-copies per offspring that asexual ones would, because they share parentage with males.
• Muller's ratchet — In asexual populations, deleterious mutations accumulate over generations; without recombination, the mutation-free class is irreversibly lost.
• Red Queen hypothesis — Sex provides genetic novelty needed to keep up with rapidly evolving parasites; named after Lewis Carroll's Red Queen ('it takes all the running you can do, to keep in the same place').

§B — Anisogamy — the foundation of male and female

ANISOGAMY = unequal gamete sizes. Females (by definition) produce few large gametes (eggs); males produce many small gametes (sperm). This asymmetry is the foundation of nearly all sex differences.

Key points

• ANISOGAMY = unequal gamete sizes; the OPERATIONAL DEFINITION of male and female (rather than secondary sex characteristics).
• Female = the sex that makes the larger, costlier gamete; male = the sex with the smaller, cheaper gamete.
• Asymmetric gamete investment leads to asymmetric reproductive strategies: females typically more selective, males typically compete for mates.
• Anisogamy evolved in the deep past as a stable evolutionary endpoint of disruptive selection on gamete size.

Key terms

• Anisogamy — Unequal gamete sizes — the foundation of biological sex differences.
• Isogamy — Equal-sized gametes — found in some algae and fungi; the ancestral state from which anisogamy evolved.

§C — Sexual selection — Darwin's second mechanism

Sexual selection is selection on traits that increase MATING success rather than survival. It explains otherwise-puzzling traits (peacock tails, antlers) that reduce survival.

Key points

• SEXUAL SELECTION = selection arising from competition for mates, distinct from survival selection (though both fall under 'natural selection' in a broad sense).
• INTRASEXUAL selection (male-male competition): direct contests between same-sex individuals for access to mates. Produces weapons, large body size in the more competitive sex.
• INTERSEXUAL selection (mate choice, typically female choice): one sex chooses mates based on signal traits. Produces ornaments — peacock tails, bird songs.
• Sexual selection can OPPOSE survival selection: showy ornaments attract mates AND predators. Net fitness = mating gain − survival cost.
• FISHERIAN RUNAWAY: an arbitrary preference for a trait can amplify itself if 'sexy' offspring inherit both the trait and the preference (positive feedback).

Key terms

• Sexual selection — Selection for traits increasing mating success rather than survival.
• Intrasexual selection — Same-sex competition for mates (e.g., male-male combat).
• Intersexual selection — Mate choice — one sex selects partners based on signal traits.
• Sexual dimorphism — Difference in form between males and females; often a product of sexual selection.

§D — Sexual conflict and sperm competition

Male and female evolutionary interests often diverge. Sexual CONFLICT — selection pressures pulling males and females in different directions — drives much of the elaborate biology of mating.

Key points

• SEXUAL CONFLICT: males benefit from many matings; females typically benefit from FEW matings with HIGH-quality partners. Their interests diverge.
• SPERM COMPETITION: when females mate with multiple males, sperm from different males compete to fertilize her eggs. Drives evolution of large testes, long sperm, mating plugs, and sperm-removal devices.
• Cryptic FEMALE choice: females can bias paternity post-copulation through reproductive-tract physiology.
• ANTAGONISTIC COEVOLUTION: an arms race within species — male traits coercing females, female traits resisting. Examples: ducks (sexually antagonistic genital coevolution), bedbugs (traumatic insemination).
• Sexual conflict can drive RAPID evolutionary change in reproductive structures.

Key terms

• Sexual conflict — Mismatch between the reproductive interests of males and females, generating opposing selection pressures.
• Sperm competition — Competition between sperm from different males to fertilize the same female's eggs.
• Cryptic female choice — Female biasing paternity after mating, via reproductive-tract mechanisms.
• Antagonistic coevolution — Reciprocal evolutionary change between sexes (or between species) driven by conflicting interests.

L12 · Life History Evolution (Ch 12)

Life history is the schedule of an organism's birth, growth, reproduction, and death. Selection cannot maximize all these at once — trade-offs between traits force compromises, and the form those compromises take depends on the environment.

§A — Trade-offs in energy allocation

An organism has finite resources and time. Energy spent on growth cannot be spent on reproduction; reproduction now reduces resources available for survival and future reproduction.

Key points

• Life-history trade-offs: growth vs. reproduction, current vs. future reproduction, offspring number vs. offspring size.
• These trade-offs are physiological constraints — selection cannot break them, only navigate them.
• OPTIMAL allocation depends on the environment: high adult survival favors investment in future reproduction; low adult survival favors current reproduction.
• Reaction norms for life-history traits show context-dependent allocation.

Key terms

• Life history — The schedule of major events in an organism's life — age at maturity, fecundity, longevity, reproductive timing.
• Trade-off — An inverse relationship between two fitness-related traits enforced by limited resources or physiological constraints.
• Reproductive effort — The fraction of an organism's resources allocated to current reproduction vs. survival/future reproduction.

§B — Extrinsic mortality and life-history strategies

Extrinsic mortality — the rate at which adults die from external causes (predation, disease) — is the single most important determinant of optimal life-history strategy.

Key points

• HIGH extrinsic mortality (e.g., heavy predation) → favors EARLY reproduction, MANY offspring, short lifespan ('live fast, die young').
• LOW extrinsic mortality → favors DELAYED reproduction, FEWER but better-provisioned offspring, longer lifespan.
• Classic comparison: opossums on predator-rich mainland mature earlier and produce more offspring than opossums on predator-free islands.
• These life-history shifts evolve quickly — within tens of generations under strong selection.

Key terms

• Extrinsic mortality — Death from external causes (predation, disease, accident) — outside the organism's physiological control.
• Intrinsic mortality / senescence — Death from internal causes (aging, organ failure) — physiological decline.

§C — Theories of senescence — why we age

Why doesn't selection prevent aging? Two main answers, both rooted in the fact that selection becomes weaker at older ages.

Key points

• MUTATION ACCUMULATION (Medawar): late-life deleterious mutations escape selection because they affect individuals who've already reproduced. Such mutations accumulate.
• ANTAGONISTIC PLEIOTROPY (Williams): some genes have BENEFICIAL effects early in life and HARMFUL effects late. Selection favors them on net because early-life effects on reproduction outweigh late-life costs.
• DISPOSABLE SOMA (Kirkwood): organisms allocate finite resources between somatic maintenance (long life) and reproduction (more offspring). Investment in soma reduces fecundity.
• These theories are not mutually exclusive — all may contribute.

Key terms

• Senescence — Age-related decline in survival and reproduction; aging.
• Mutation accumulation theory — Late-acting deleterious mutations escape selection and accumulate, causing senescence.
• Antagonistic pleiotropy — A single gene with beneficial early-life effects and harmful late-life effects; selection favors the early benefit.
• Disposable soma theory — Trade-off between somatic maintenance and reproduction; selection favors reproduction at the cost of long-term cell repair.

§D — Age at maturity and offspring size

When to start reproducing, and how to package each offspring, are central life-history decisions. Both depend on environmental risk and resource availability.

Key points

• AGE AT MATURITY: earlier maturity → reproduce sooner but at smaller size (less invested in growth). Optimal age depends on growth rate, mortality risk, and reproductive payoff.
• OFFSPRING SIZE-NUMBER trade-off: a fixed reproductive budget can be split into MANY SMALL offspring (high mortality of each, but bet-hedging) OR FEW LARGE offspring (each well-provisioned, higher per-offspring survival).
• r-selected species (high mortality, ephemeral resources): many small offspring, early maturity. (Mice, weeds.)
• K-selected species (stable, competitive environments): few large offspring, delayed maturity, parental care. (Elephants, oaks.)

Key terms

• Age at maturity — Age at which an individual first reproduces.
• r/K selection — Classical (and somewhat dated) framework distinguishing fast-living, high-fecundity (r) from slow-living, low-fecundity (K) life histories.

§E — Case study — Seychelles warblers

The Seychelles warbler (Acrocephalus sechellensis) population on Cousin Island provides one of the best documented life-history datasets, including sex-biased dispersal driven by territory quality.

Key points

• Seychelles warblers cooperatively breed: offspring help parents raise subsequent broods.
• Territory quality varies dramatically; high-quality territories support more breeding success.
• FEMALE OFFSPRING are more likely to remain on natal high-quality territories as helpers; MALE OFFSPRING disperse — sex-biased dispersal driven by territory value.
• Demonstrates how ecological constraints (territory quality, helping opportunities) shape life-history decisions about dispersal, helping, and breeding.

Key terms

• Cooperative breeding — A breeding system in which adult helpers (often offspring from previous broods) assist parents in raising young.
• Sex-biased dispersal — One sex disperses from the natal site more than the other; common in birds and mammals.

L13 · Evolution of Social Behavior (Ch 16)

How can selection favor cooperative or self-sacrificing behavior when those behaviors apparently lower the actor's fitness? The answer — kin selection, inclusive fitness, evolutionarily stable strategies — is a foundational result in modern evolutionary biology.

§A — Individual vs. group selection

An old idea: traits evolve 'for the good of the species' or 'good of the group.' Modern evolutionary biology rejects naïve group selection — selection acts predominantly at the level of individuals (or genes), and selfish individuals usually outcompete cooperative ones.

Key points

• GROUP SELECTION (naïve form): traits spread because they benefit the group, even at individual cost.
• Problem with naïve group selection: a 'cheater' that benefits from cooperation without contributing has higher individual fitness. Cheaters spread; cooperation collapses.
• Modern view: selection mostly works at the individual or genic level; apparent group benefits arise from individual benefits.
• Special conditions (frequent population extinctions, strict subdivision) can permit some forms of group-level selection, but it's the exception.

Key terms

• Group selection — Selection in which differential reproduction of GROUPS (not individuals) drives evolutionary change.
• Selfish gene perspective — Dawkins's framing: selection favors alleles that increase their own propagation, even if that means promoting altruistic behavior toward genetic relatives.

§B — Kin selection and inclusive fitness

Hamilton's insight: an altruistic gene can spread if its bearers help GENETIC RELATIVES, who share copies of the gene. The propagation of the gene matters, not the survival of the individual.

Key points

• HAMILTON'S RULE: rB > C, where r = coefficient of relatedness, B = benefit to recipient, C = cost to actor. Altruism evolves when rB > C.
• Coefficient of relatedness r: parent-offspring r = 0.5; full siblings r = 0.5; half-siblings r = 0.25; first cousins r = 0.125.
• INCLUSIVE FITNESS = direct fitness (own offspring) + indirect fitness (extra offspring of relatives that wouldn't have existed without your help, weighted by relatedness).
• Eusociality (worker bees, ants, naked mole-rats) is the extreme case — workers forgo reproduction entirely to raise relatives.
• Haplodiploidy in hymenopterans: sisters share r = 0.75 (more than they'd share with their own offspring, r = 0.5), favoring sister-helping behavior. Disputed as a complete explanation, but suggestive.

Key terms

• Hamilton's rule — Altruism is favored when rB > C; rB is the indirect benefit through relatives, C is the cost to the actor.
• Coefficient of relatedness (r) — Probability that two individuals share a given allele by recent common descent.
• Inclusive fitness — Direct fitness from own offspring plus indirect fitness gained through helping relatives reproduce.
• Eusociality — Highly social organization with reproductive division of labor — workers forgo reproduction to raise relatives.

§C — Evolutionarily stable strategies (ESS)

An ESS is a behavioral strategy that, once common in a population, cannot be invaded by a rare alternative. ESS reasoning predicts which behaviors persist under frequency-dependent selection.

Key points

• ESS = a strategy that, when adopted by most of a population, cannot be invaded by a rare alternative — a Nash equilibrium in evolutionary terms.
• ESS depends on FREQUENCY-DEPENDENT payoffs: the fitness of a strategy depends on what other strategies are common.
• Hawk-Dove game: pure-Hawk is unstable (Hawks fight, costs mount); pure-Dove is unstable (a rare Hawk exploits all the Doves). Mixed equilibrium emerges.
• ESS is solved with game-theory tools — payoff matrices and frequency-dependent fitness.

Key terms

• Evolutionarily Stable Strategy (ESS) — A strategy that, when adopted by most members of a population, cannot be invaded by any rare alternative strategy.
• Frequency-dependent selection — Fitness of a strategy depends on its frequency in the population — common strategies may be at advantage or disadvantage.
• Hawk-Dove game — Classic ESS model: aggressive (Hawk) and submissive (Dove) strategies in resource competition; equilibrium is a mix.

§D — Side-blotched lizards — rock-paper-scissors in nature

Male side-blotched lizards (Uta stansburiana) come in three throat-color morphs that play a rock-paper-scissors mating game — a real-world demonstration of frequency-dependent ESS dynamics.

Key points

• Three male morphs: ORANGE (aggressive, defends large territories with many females), BLUE (defends small territories with one mate, vigilant against sneakers), YELLOW (sneaker, mimics females, sneaks copulations).
• Orange beats Blue (Orange's aggression overwhelms Blue's small-territory defense).
• Blue beats Yellow (Blue's vigilance catches Yellow sneakers).
• Yellow beats Orange (Orange spreads thin defending many females; Yellow sneaks in unnoticed).
• Result: frequencies of the three morphs cycle over years — no single ESS, but a stable cyclic dynamic.

Key terms

• Side-blotched lizard (Uta stansburiana) — Lizard species with three male throat-color morphs displaying rock-paper-scissors mating dynamics.

§E — Cooperation among non-kin — reciprocity, reputation, byproducts

Cooperation among unrelated individuals does occur. The mechanisms are different from kin-selected altruism — reciprocity, reputation, partner choice, or mutual benefit.

Key points

• DIRECT RECIPROCITY: 'I help you now if you help me later.' Requires repeat interaction and memory.
• INDIRECT RECIPROCITY: helping confers a reputation that attracts future help from third parties. Requires public visibility.
• MUTUALISTIC BYPRODUCT: cooperation costs nothing (or even directly benefits the actor) — pure self-interest aligns.
• Cheating is always tempting; cooperation among non-kin is fragile and requires enforcement, sanctions, or partner choice to persist.

Key terms

• Direct reciprocity — Cooperation maintained by tit-for-tat exchange between two individuals over repeated interactions.
• Indirect reciprocity — Cooperation maintained because helpful behavior earns a reputation that attracts future help from others.